Chronic inflammatory diseases, such as rheumatoid arthritis, Crohn's disease, multiple sclerosis and atherosclerosis, are diseases in which the body's immune system produces anti-self (ie. autoimmune) responses. In the case of rheumatoid arthritis, the clinical syndrome is characterised by relapsing/remitting inflammation within the synovial membrane, associated with progressive, erosive destruction of adjacent cartilage and bone. On a cellular level, these events are coincident with chronic infiltration of the synovial membrane with T cells, plasma cells and macrophages.
There is mounting evidence to implicate T cells in the initiation and perpetuation of chronic inflammatory diseases such as rheumatoid arthritis (RA). For example, patients with RA have been found to carry a specific polymorphism in the class II major histocompatibility complex (MHC), which is believed to render them genetically predisposed to developing RA (Panayi et al., 1992, Arthritis Rheum. 35:729-735). A key function of class II MHC is to present antigen to a subpopulation of T cells, termed T helper cells, which are characterised by CD4+ surface markers. It is proposed that antigen presentation results in clonal expansion and activation of this population of T cells, which in turn leads to stimulation of other populations of synovial cells, such as macrophages, and the consequent release of pro-inflammatory cytokines. To date, however, the nature of the environmental stimulus that triggers RA remains unknown.
In addition to a role in initiation of RA, T helper cells are also proposed to be important in the perpetuation of this disease, either by becoming activated against self-proteins (e.g. peptides derived from the degradation of MHC molecules and/or structural proteins in the joint) or by being re-exposed to the initiating antigen. This further activation of T cells is suggested to result in the sustained release of pro-inflammatory cytokines, as well as enzymes that mediate destruction of the cartilage and bone.
As a consequence of the growing evidence of an involvement of T cells in the pathogenesis of RA, several treatments of this disease have been developed which target T cells via their surface markers, for example anti-CD4 antibody therapy, anti-CD52 antibody therapy, anti-CD5 antibody therapy and anti-interleukin-2 receptor antibody therapy (e.g. van der Lubbe et al., 1995, Arthritis Rheum. 38:1097-1106; Weinblatt et al., 1995, Arthritis Rheum. 38:1589-1594; Olsen et al, 1996, Arthritis Rheum. 39:1102-1108; Moreland et al., 1995, Arthritis Rheum. 38:1177-1186). However, the results of clinical trials with such therapies have been largely disappointing, as a result of a lack of efficacy and/or the presentation of toxicity-related side effects. These limitations are thought to be due, at least in part, to the lack of specificity of the therapies for the particular population of T cells responsible for inducing production of pro-inflammatory cytokines by macrophages in RA joints.
An alternative approach for developing new treatments of RA has been to target the pro-inflammatory cytokines produced in the synovium by macrophages in response to T cell activation. Although several different pro-inflammatory cytokines may be released from macrophages following T cell activation in RA tumour necrosis factor α (TNFα) has attracted particular attention. The importance of TNFα was initially demonstrated using dissociated rheumatoid joint cell cultures (Brennan et al., 1989, Lancet 2(8657): 244-247) and subsequently confirmed in animal models of arthritis (Thorbecke et al., 1992, Proc. Natl. Sci. USA 89:7375-7379; Williams et al., 1992, Proc. Natl. Sci. USA 89:9784-9788). The findings of these pre-clinical studies led to successful clinical trials of anti-TNFα antibody therapy, establishing the importance of TNFα as a therapeutic target (e.g. Elliott et al., 1993, Arthritis Rheum. 36:1681-1690; Elliott et al., 1994, Lancet 344:1105-1110; Elliott et al., 1994, Lancet 344:1125-1127).
In RA joints, the cells responsible for the majority of TNFα production are macrophages (Chu et al., 1991, Arthritis Rheum. 34:1125-1132). Hence, there is much interest in understanding the mechanisms underlying the regulation of TNFα production in cells of this lineage. Available data suggest that both water-soluble factors and cell-cell interactions may be involved in mediating T cell-induced production of TNFα by macrophages/monocytes.
Sebbag et al. (1997) Eur. J. Immunol. 27:624-632 recently demonstrated that cytokine stimulation (using IL-15 alone, or a cocktail of IL-6, TNFα and IL-2) could activate a specific subset of T helper cells termed cytokine stimulated T cells or ‘Tck cells’ (formerly ‘Tcy cells’), which in turn could induce TNFα production (but not IL-10 production) in monocytes. Stimulation of conventional T cell receptor-stimulated T cells (‘Ttcr cells’) using anti-CD3 antibodies also induced TNFα production by monocytes, in addition to the production of an anti-inflammatory cytokine, IL-10 (Parry et al., 1997, J. Immunol. 158:3673-3681). In the case of both Tck and Ttcr cell stimulation, TNFα production was found to be dependent on cell-cell interactions between T cells and macrophages. On the basis of these findings, Sebbag et al. postulated that Tck cells might contribute to the production of pro-inflammatory cytokines in RA synovial tissue, thus contributing to the relative imbalance of pro-inflammatory cytokines (e.g. TNFα) over anti-inflammatory cytokines (e.g. IL-10) in such tissue.
Interleukin-15 (IL-15) has also been implicated in the production of TNFα in RA. This pro-inflammatory cytokine, known to be present in RA synovium (McInnes et al, 1996, Nature Medicine 2:175-182), has been shown to activate peripheral blood T cells which, in turn, are able to induce TNFα production in U937 cells and adherent RA synovial cells in a contact-dependent manner (McInnes et al, 1997, Nature Medicine 3(2): 189-195).
To date, however, the identity of the T cell population(s) that mediate(s) the production of pro-inflammatory cytokines in RA synovial tissue remains unclear. Furthermore, a means of selectively targeting such cells is absent. Hence, the present invention seeks to provide a method of identifying compounds with efficacy in the treatment of a chronic inflammatory disease, for example compounds which selectively target, either directly or indirectly via cytokine-stimulated T cells, macrophages/monocytes responsible for the production of pro-inflammatory cytokines in RA synovial tissue.